In a device for purifying exhaust gas of an internal combustion engine equipped with a diesel particulate filter (hereinafter referred to as “DPF”), a selective catalytic reduction DeNOx device (hereinafter referred to as “SCR device”) or a NOx storage-reduction catalyst (hereinafter referred to as “NSR catalyst”), it is normal to arrange an additive feeding device for adding a reducing agent in the exhaust passage upstream of the DPF, SCR device or NSR catalyst so that the exhaust gas is purified upon reacting with the additive fed from the additive feeding device, in the DPF, SCR device or NSR catalyst. To prevent the pass-by of the additive through the DPF, SCR device or NSR catalyst and to effectively utilize the additive, it is preferable to mix the additive and the exhaust gas together to a sufficient degree and to widely diffuse the additive over the entire cross section of the exhaust passage.
Therefore, a diffuser board has been arranged downstream of the additive feeding device but upstream of the DPF, SCR device or NSR catalyst to mix the additive with the exhaust gas so that the additive was widely diffused over the whole cross section of the exhaust passage. However, means for arranging the diffuser board has resulted in an increase in the pressure loss of the exhaust gas flow.
Japanese Unexamined Patent Publication JP-A-2007-205308 discloses a device for purifying exhaust gas of an internal combustion engine by feeding a purifying agent from the upstream of the device for purifying exhaust gas provided in the engine exhaust passage in order to purify specific components in the exhaust gas, wherein provision is made of a tilted portion for gradually decreasing the sectional area of the exhaust passage, and the purifying agent is injected in the radial direction of the exhaust passage from an injection port disposed near the central portion of the exhaust passage near a portion where the exhaust gas converges through after passing through the tilted portion, in order to improve the diffusion of the purifying agent.